A variety of devices designed to reflect unauthorized tampering have been described previously. A number of proposals for tamper-proof labels, for example, which are designed for attachment to other objects, have been described.
U.S. Pat. No. 1,003,443 to Erickson discloses a stamp or label having complementing films, one of which adheres to a receiving surface to discourage second use of the stamp or label.
U.S. Pat. No. 3,487,567 to Waybright discloses an emblem or label for a vehicular window having an adhesive on one side of a transparent sheet and an indicia on the other, with an opaque layer carrying special indicia affixed to the transparent sheet.
U.S. Pat. No. 3,631,617 to Pekko discloses a tamperproof label having a transparent self-supporting film which is imprinted with a first visual indicia. There is a masking surface which is in contact with at least part of the surface of the transparent substrate. The masking surface is imprinted with a visual indicia. An adhesive layer is applied having a first surface in contact with the substrate and the masking surface, the second surface of the adhesive layer contacting a substrate. It is a goal of the label of this patent to provide a visual imprinting when the label is sealed in place. When the label is applied to a substrate and thereafter removed, a portion of the indicia remains on the substrate and a portion is removed with the film.
U.S. Pat. No. 4,082,873 to Williams discloses switchproof laminated labels having a transparent plastic film on which an indicia is imprinted in reverse and which is coated with an adhesive. The printing has a greater affinity for the adhesive than the film so that when mounted on an object and then removed, a tamper indicating amount of the printing remains on the object.
U.S. Pat. No. 4,180,929 to Schultz discloses tamper indicating multilaminate labels having two sets of indicia, one of which is rendered visible upon tampering and a second which is visible regardless of tampering.
U.S. Pat. No. 4,608,288 to Spindler discloses tamper proof labels having an irreversibly stretchable cover foil adhesively joined to a base foil with varying degrees of adhesion.
A number of references also describe tamper indicating tapes which are applied to containers and provide means indicating when the seal has been broken.
U.S. Pat. No. 3,923,198 to Brochman discloses a tape which opacifies when stressed.
U.S. Pat. No. 3,935,960 to Cornell discloses a tamper indicating tape having encapsulated dye in a binder material.
U.S. Pat. No. 4,557,505 to Schafer discloses a tape which opacifies when stressed and which carries contrasting colored indicia.
U.S. Pat. No. 4,652,473 to Han discloses a tape of two different materials which is heat shrunk onto a container.
Such tapes and labels must be applied to the outside of containers in a separate step. Moreover, these devices can be removed in the course of intentional tampering and the tampering indicia then covered, as with opaque tape, so to conceal the evidence of tampering. Various attempts have been made to incorporate tamper indicating means into the actual product so as to eliminate the need for affixing a label or tape.
U.S. Pat. No. 2,181,252 to Vogel discloses a safety envelope having two flaps, one of which carries a printing in ink which is soluble in the same solvents as is the envelope adhesive.
U.S. Pat. No. 3,933,304 to Judd discloses bags for bank notes which are heat sealed at a position adjacent to an area of printing so that the bags cannot be opened without mutilating the printing.
U.S. Pat. No. 4,505,399 to Weiner discloses a closure which includes a material sensitive to ambient conditions such as light or oxygen.
U.S. Pat. No. 4,718,553 to Adamoti discloses tamper evident packaging for cartons in which a transparent film overwraps the carton and is secured to ink on the carton in selected different patterns.
U.S. Pat. No. 4,721,217 to Phillips discloses a cap having a pair of layers providing a color shift which is destroyed or modified when the layers are separated.
U.S. Pat. Nos. 4,709,396 and 4,709,397 to Voshall disclose a tamper evident envelope in which a pressure sensitive seal along one edge contains a hidden message. Two cohesive layers, one continuous and the other non-continuous, are employed with the latter creating voids appearing as reverse drop-out patterns when the two cohesive layers are separated.
These devices generally involve either complicated manufacturing steps or utilize relatively expensive materials. This is particularly true for devices in which the tamper-indicating means are incorporated into the actual product. Devices such as those disclosed by Voshall in U.S. Pat. Nos. 4,709,396 and 4,709,397, for example, require two cohesive layers which must be separated prior to use, as well as inner paper liners to conceal the contents of the package since the outer sheet must be transparent to reveal the tampering indicia. In addition, many tamper indicating devices, particularly tapes, so complicate the overall packaging that the intended user may encounter difficulty simply trying to open the package, often a serious problem with the elderly.